1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube of a shadow mask type capable of a high resolution display for use in, for instance, a terminal display device in a computer system.
2. Description of the Prior Art
FIG. 1 shows a prior art color cathode ray tube of a shadow mask type for television. A fluorescent surface 3 is deposited and formed on the inside surface of a panel 1 on which an image is displayed. The fluorescent surface 3 includes a large number of fluorescent stripes of three colors each of which continuously extends in the vertical direction relative to scanning lines of electron beams discharged from electron guns 2. A nearly cone-shaped funnel portion 5 having a tubular neck portion 4 is connected to the panel 1 to form a vacuum vessel. A unit of three electron guns 2 disposed in line is mounted in the neck portion 4. A deviation yoke 6 for making electron beams deviate is mounted on the circumference of the funnel portion 5. A shadow mask assembly 11 comprising a shadow mask 8 and a frame 10 is mounted inside of the panel 1. The shadow mask 8 is provided with slots 7 for allowing an electron beam selectively to permeate correspondingly to the fluorescent surface 3. An inside magnetic shield 12 extending toward the electron guns 2 along the inside surface of the funnel portion 5 is directly or indirectly fixed to the frame 10 of the shadow mask structure 11.
In the shadow mask type color cathode ray tube having the above construction, three electron beams discharged from the electron guns 2 are made to deviate with horizontal and vertical magnetic fields generated by the deviation yoke 6 so as to scan the entire of the fluorescent surface 3, as shown in FIGS. 1 and 2. The electron beams run into the fluorescent stripes of the corresponding colors through the corresponding slots 7 of the shadow mask 8, respectively. The fluorescent matters in the stripes are then excited and radiate to display a color image.
Generally in the slot type shadow mask 8, a large number of nearly parallel slot rows in each of which rectangular slots 7 are disposed in the vertical direction of the imaging surface at a constant vertical pitch, for instance, Pv=0.5 mm to 1.0 mm, is arranged in a regular manner at a horizontal pitch, for instance, Ph=0.5 mm to 1.2 mm, as shown in FIG. 3. A bridging portion 9 is formed between each vertically neighboring two slots 7 for maintaining the mechanical strength of the shadow mask 8. The vertical sizes of the openings of the slots 7 are nearly equal to each other. The slot rows are designed so that each bridging portion of each slot row is positioned at the vertical center of the nearest slots of both neighboring slot rows.
On the other hand, in a shadow mask type color cathode ray tube for a high resolution display, according to an in-line electron guns 2, a circular hole type shadow mask 18 in which the pitch P of the shadow mask is almost 0.2 mm to 0.3 mm and the size of each circular opening 17 is almost 0.08 mm to 0.14 mm is mostly used in combination with a dot type fluorescent surface 13, as shown in FIGS. 4 and 5.
The combination of a slot type shadow mask 8 and a stripe type fluorescent surface 3 has an advantage that a bright picture is obtained because a large aperture rate of the shadow mask can generally be obtained in comparison with the combination of a circular hole type shadow mask 18 and a dot type fluorescent surface 13. Thus, the former combination is mainly used for television as described above. When the pitch is decreased, however, a difficulty in press molding of the shadow mask due to an anisotropy of the shadow mask pattern, a difficulty in manufacturing a color cathode ray tube due to a lack of the mechanical strength of the shadow mask, or the like become remarkable. For these reasons, the former combination has scarcely been used for a high resolution display indispensable to a fine pitch. In contrast to this, the combination of a circular hole type shadow mask 18 and a dot type fluorescent surface 13 has no remarkable problem in manufacturing even in the case of a fine pitch because of the isotropy of the shadow mask pattern. This combination has thus exclusively been used for high resolution display.
On the other hand, saving power in a color cathode ray tube is recently required for saving power consumption in a color display device. For instance, saving power in a color cathode ray tube is required for the Energy Star Programme promoted by the United States Environment Protect Department, or for avoiding regulations of measures for high mode waves such as electromagnetic waves mainly promoted in Europe, especially in Sweden.
Although some measures for saving power of a color cathode ray tube can be called to mind, it is most effective among them to decrease the power for deviation. Since it is considered that the deviation power is theoretically almost in proportion to the square of the neck diameter, the deviation power is decreased by about 40% if the neck diameter is changed from conventionally 29.1 mm to 22.5 mm. Even when the neck diameter is changed to 24.3 mm, it is expected to decrease the deviation power by about 30%. For this reason, the measure that the neck diameter is decreased to decrease the deviation power is generally taken.
As for the standard neck diameter, in color cathode ray tubes for high resolution display, the neck diameter of 29.1 mm is currently used as a standard. In contrast to this, a small neck diameter of 24.3 mm or 22.5 mm begins to be used as a practical standard for the purpose of saving the deviation power.
When the neck diameter of a cathode ray tube is decreased, because electron guns sealed in the neck portion become small-sized, the degradation of focusing with decreasing the aperture size of an electron lens comes into the first question. When the largest size of electron guns within the limited space in the neck tube are designed for improving focusing, the convergence drift with orbital changes of both side beams due to charges in the inside wall of the neck portion comes into a new question. Even if the problems of focussing and the convergence drift are resolved, because the landing shift of an electron beam due to the terrestrial magnetism with the increase of the distance (Q value) between a fluorescent surface and a shadow mask corresponding to the decrease of the caliber (S value) of electron guns, the deterioration of the margin of the color purity comes into a new question.
More detailedly, we tried to develop a saving power tube in which the neck diameter is decreased to 22.5 mm in a color cathode ray tube of a circular hole type shadow mask type for a high resolution display. Although focussing and the convergence drift could meet standard levels, respectively, the problem of the color purity margin was left. That is, when the neck diameter is decreased, the caliber of an sealed electron gun is also decreased. As a result, the distance (S value) between each neighboring electron beams of three electron guns is decreased. When the S value is decreased, the Q value which is the distance between the fluorescent surface and the shadow mask must be increased more than the conventional value for densely disposing on the fluorescent surface three electron beams of blue, green and red passed through openings of the shadow mask. As a result, the movements of electron beams due to the terrestrial magnetism was also increased and the color purity margin was decreased.
Although changes in design of the shape of the inside magnetic shield in a color cathode ray tube of a circular hole type shadow mask type were considered for decreasing the landing shift of an electron beam due to the terrestrial magnetism which was increased as the result of decreasing the S value of electron guns and increasing the distance between the fluorescent surface and the mask, there was a limit in decreasing the absolute value of the landing shift of an electron beam due to the terrestrial magnetism so a sufficient decrease was not attained. As a result, although the performances in focusing and the convergence drift as those of a conventional tube could be obtained, there was left a problem that the color purity margin did not always reached to that of the conventional tube. This is shown more concretely by numerical values as the following table 1.
TABLE 1 ______________________________________ comparison of properties due to differences of neck diameter, Q value and electron guns landing elect- shift pitch ron by and guns ter- type of neck effec- restrial shadow dia- Q tive magne- Cg re- mask meter value S value tism drift marks ______________________________________ 0.27 mm 29.1 8.5 mm 5.08 mm 0.035 mm 0.05 OK pitch mm mm circu- 22.5 9.6 4.50 0.040 0.10 NG lar hole 22.5 10.4 4.10 0.043 0.05 NG type ______________________________________
The landing shift by the terrestrial magnetism is represented by a changing amount of the landing position of an electron beam on a fluorescent surface after demagnetization when the tube face of a color cathode ray tube is directed from the south to the north. In the case of a circular hole type shadow mask tube with 0.27 mm pitch, this value is preferably 0.040 mm or less so it is targeted in design. As for this landing shift by the terrestrial magnetism, a conventional tube of the neck diameter of 29.1 mm was passed because its landing shift by the terrestrial magnetism was 0.035 mm. In the case of a tube of the neck diameter of 22.5 mm and the effective S value of 4.50 mm, although its landing shift by the terrestrial magnetism was 0.040 mm so it was only just passed, its convergence drift was larger as 0.1 mm so the tube was not passed as a whole. A tube of the effective S value of 4.10 mm was not passed because its landing shift by the terrestrial magnetism was 0.043 mm which exceeds the targeted value in design.
Recently, it has been found that even the combination of a slot type shadow mask and a stripe type fluorescent surface in a color cathode ray tube for a high resolution display can industrially be realized if the pitch or the like of the shadow mask is limited (the Japanese Patent Application No. 14856/1986).
In the case of this color cathode ray tube of the slot type shadow mask type, in comparison with a circular hole type shadow mask type color cathode ray tube having the equal resolution, the Q value is small as nearly half so it is hard to be affected by the terrestrial magnetism. In addition, in the case of the slot type shadow mask type color cathode ray tube, the S value of electron guns is decreased and the Q value is increased and there is a possibility substantially to decrease the landing shift by the terrestrial magnetism increased as a result of the decreased S value and the increased Q value. That is, even if it is difficult considerably to decrease the absolute value of the landing shift by the terrestrial magnetism, there is a possibility that the shape of the inside magnetic shield is changed in design so that the vertical component of the landing shift by the terrestrial magnetism is increased and the horizontal component is decreased. An increase of the landing shift of an electron beam in the vertical direction substantially never affects the color purity margin because the electron beam moves along a fluorescent stripe. A substantial affection appearing on a picture is only by the horizontal movement which is decreased so it becomes possible to prevent the color purity margin from deteriorating.